The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed by Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 ( 1)
where
P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA1 P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium.
.beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi.`.sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3 ( 2)
where
.chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi..sup.(1) E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.3 E.sup.3.
To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu.
A significant difficulty encountered in finding suitable molecular dipoles for second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies).
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular dipoles are placed in an electric field.
Second order polarization (.chi..sup.(2) E.sup.2) has been suggested to be useful for a variety of purposes, including optical rectification (converting electromagnetic radiation input into a DC output), generating an electro-optical (Pockels) effect (using combined electromagnetic radiation and DC inputs to alter during their application the refractive index of the medium), phase alteration of electromagnetic radiation, and parametric effects, most notably frequency doubling, also referred to as second harmonic generation (SHG).
For a number of years the materials employed for achieving second order polarization effects were noncentrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. Interest in nonlinear optical properties has increased in recent years, driven primarily by the emergence of optical telecommunications, but also stimulated by a broader need to raise optical manipulation capabilities closer to parity with those employed in electronics. This has resulted in an unsatisfied need for higher performance materials.
D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23, 1984, 690, reports second order polarization susceptibilities, .chi..sup.(2), achieved with a variety of organic molecular dipoles. The molecular dipoles reported are comprised of an electron acceptor moiety bonded to an electron donor moiety by a linking moiety providing a conjugated .pi. bonding system for electron transfer. Specific electron donor moieties disclosed are dimethylamino, 2- or 4-pyridyl, 2-quinolinyl, and 2-benzothiazolyl. Specific conjugated .pi. bonding systems reported are phenylene and combinations of ethylene (vinylene) and phenylene moieties. Specific electron acceptor moieties disclosed are oxo, cyano, and nitro.
Zyss, "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, 1, 1985, 25, discloses a variety of molecular dipole structures for nonlinear optics.
Ulman et al U.S. Pat. No. 4,792,208 demonstrates organic molecular dipoles having high (&gt;10.sup.-30 esu) second order hyperpolarizabilities (.beta.) and capable of being polar aligned to produced films exhibiting high (&gt;10.sup.-9 esu) second order polarization susceptiblities (.chi..sup.(2)). The substitution of sulfonyl as an electron acceptor moiety for the oxo, cyano, and nitro electron acceptor moieties previously known to the art offers a variety of advantages. None of the oxo, cyano, or nitro moieties can be chemically substituted without destroying their essential electronic properties. On the other hand, the sulfonyl moiety of Ulman et al requires by definition a hydrocarbon substituent, which can be further substituted with functional groups, if desired. Thus, the sulfonyl electron acceptor moiety offers much greater synthetic freedom for controlling the physical properties of the molecular dipole for optimum utilization. The substitution of sulfonyl dipoles for oxo, cyano, and nitro dipoles can extend optical utility to different wavelength regions of the spectrum by being more transparent to input electromagnetic radiation, output radiation--e.g., second harmonic radiation, or a combination of both. The sulfonyl dipoles offer a broader range of solvent and binder compatibilities for achieving the required polar alignments for useful effects produced by second order polarization in optical articles. Sulfonyl substitution to achieve optimized physical compatibility with other materials encountered in optical article fabrication is readily achieved. For the fabrication of Langmuir-Blodgett (LB) films the sulfonyl group can be chosen to exhibit either a hydrophilic or hydrophobic characteristic. Additionally, the sulfonyl group can be chosen to act as a linking group to a polymer backbone, if desired. By employing a sulfonyl electron acceptor group in combination with a hydrocarbon substituted electron donor group it is apparent that both ends of the dipolar molecule can be optimized for the construction of polar aligned molecular dipoles.
Compounds in which a sulfonyl group is halo-substituted are generally known in the art. Illustrative compounds are illustrated by the following:
C-1 R. J. Koshar and R. A. Mitsch, "Bis(perfluoroalkylsulfonyl)methanes and Related Disulfones, J. Org. Chem., 1973, 38, 3358;
C-2 Koshar U.S. Pat. No. 3,758,593;
C-3 Koshar et al U.S. Pat. No. 3,776,960;
C-4 Koshar et al U.S. Pat. No. 3,794,687;
C-5 Koshar U.S. Pat. No. 3,932,526;
C-6 Coles et al U.S. Pat. No. 3,933,914;
C-7 Koshar U.S. Pat. No. 3,984,357;
C-8 Koshar et al U.S. Pat. No. 4,053,519;
C-9 Skoog U.S. Pat. No. 4,018,810;
C-10 P. I. Ogoiko, V. P. Nazretyan, A.Ya. Il'chenko, and L. M. Yaguol'skii, "Perfluoroalkylsulfonylacetic and Perfluoroalkylsulfonylmalonic Esters", J. Org. Chem. USSR, 1980, 16, 1200;
C-11 L. M. Yagupol'skii and L. Z. Gandel'sman, "Effect of the Trifluoromethylsulphonyl Group on the Colour of Dimethylaminoazo Dyes," Dyes and Pigments, 1982, 3, 1;
C-12 Leichter et al European Patent Application No. 0,058,839, published Apr. 16, 1986; and
C-13 P. D. Ries and C. J. Eckhardt, "Observation of the H Band in the Crystal Spectrum of 4-{4,4-Bis[(trifluoromethyl)sulfonyl]-1,3-butadienyl}-N,N-dimethylbenzenea mine (FSMB)", Chem. Phys. Lett., 1988, 153, 223.
None of the compounds of C-1 to C-13 were mentioned by their authors to have utility in high second order polarization susceptibility optically active layers. Still more revealing, none or these compounds nor any compound having a sulfonyl electron acceptor moiety was discussed by Williams or Zyss, cited above, in their general discussions of organic molecular dipoles and optically active layers.